1. Field of the Invention
The invention relates generally to communications and relates more particularly to the sampling of a signal comprising a succession of transitions of level which are representative of a two-phase coding of binary information. The invention applies advantageously but not limitingly to the sampling of a video signal with a view to extracting therefrom the digital information which it contains.
2. Description of the Prior Art
Within the meaning of the present invention, a two-phase coding is a coding in which at least some of the binary information is coded by means of pulses whose pulse duration is equal to the period of transmission of this binary information, and which exhibit an inversion of polarity (transition) in the middle of the said pulse duration, that is to say in the middle of the binary information transmitted.
More precisely, the code known by those skilled in the art as the "Manchester code" may be mentioned in this regard, the latter generating transitions for each binary information element, irrespective of the sequence sent. A logic "1" in this code is coded as a rectangular pulse whose duration is equal to the period of transmission, with inversion of polarity in the middle of the bit, the first half being of positive sign. A logic "0" is coded as a pulse of identical duration but opposite polarity.
Another so-called "two-phase" code is known to those skilled in the art as the "Miller code". According to this code, a logic "1" is coded by using a pulse of the Manchester type, that is to say a rectangular pulse with inversion of polarity in the middle of the bit while a logic "0" is coded using a rectangular pulse with no change of polarity. The polarity of the pulses corresponding to the sending of a logic "1" is chosen in such a way as to guarantee continuity with the previous pulse. As far as the polarity of the pulses corresponding to the sending of a logic "0" is concerned, it ensures continuity (non-transition) after a logic "1", but it is inverted after a previous "0". Additionally, in this code, the signal contains one transition at least every two bit durations, thereby ensuring sufficient transitions for recovery of the data stream.
Several methods for sampling such a signal are currently known. A first method consists in using an analog system relying on phase-servo control and frequency-servo control, using a phase-locked loop, thus making it possible to obtain a clock signal which is servo controlled to the frequency of transmission of the data and which is in phase with the transitions of these same data. However, this method has the drawback of requiring the use of a phase-locked loop, because this is an analog system, which is difficult to manage and relatively expensive to use.
A second method consists in using a system which carries out, in a first step, the analog/digital conversion of the data and then, in a second step, the processing of its samples by one or several more or less complex signal processing algorithms. This solution, although it fully satisfies the reproducibility criterion on account of its basically digital nature, on the other hand does not actually make it possible to obtain a truly economically beneficial solution, especially when it is intended to be incorporated into industrially mass-produced systems.
Accordingly, there is a need for a method and device which overcomes these limitations.